Brightness constancy is a perceptual constancy, in which viewers perceive an object’s level of brightness relative to its surroundings or background/environment.
Image Description:
The floor seen in the bright light reflects a different amount of light than the same floor seen with darkness in a softly lighted room. Yet we perceive the floor as having the same colour in each case. This is an example of brightness constancy. Even though a persons shadow is covering the floor we still know and perceive that the floor is the same colour and brightness and has not changed.